In analytical chemistry, liquid and gas chromatography techniques, as well as supercritical fluid chromatography techniques, have become important tools in the identification of chemical sample components. The basic principal underlying all chromatographic techniques is the separation of the sample chemical mixture into individual components by transporting the mixture in a moving fluid through a porous retentive media. The moving fluid is referred to as the mobile phase and the retentive media has been referred to as the stationary phase. Generally, such techniques require the analyst to monitor and/or control the mass-flow of the mobile phase. Such monitor or control operation, typically involves measuring the mass flow of a particular fluid in the chromatographic system. To this end, several mass-flow sensing devices have been developed.
One type of mass-flow sensor which has been developed for use with Tylan Mass-Flow Meters is said to include two heated resistance thermometers mounted on a small, stainless-steel sensor tube in spaced relationship to one another. It has been stated that when gas is flowing through the tube, heat is transferred by the moVing fluid to the downstream thermometer thereby producing a signal proportional to the gas flow. Each resistance thermometer forms a part of a bridge and amplifier circuit that produces a 0 to 5 volts direct current (DC) signal proportional to the gas. See promotional literature for Tylan Mass-Flow Meters, page 1.
A similar type of mass-flow sensor is disclosed in Brooks Mass-Flow Meter descriptive literature by Brooks Instrument Division, Emerson Electric Company, 1981. The operation of the mass-flow sensor is stated as initially directing regulated heat to the mid-point of a flow-carrying sensor tube. Resistance temperature measuring sensors are stated to be positioned at equal distance points upstream and downstream of the heater. When gas is flowing through the sensor tube, the gas stream is said to carry heat from the upstream sensor to the downstream sensor. An increasing temperature difference developing between the two sensors is said to be proportional to the amount of gas flowing through the sensor. It is indicated that the temperature difference is interpreted by a bridge circuit and an amplifier provides a 0-5 volt DC output to an indicator and/or controller. Other devices of this type are described in Gallant, J., Thermal Mass-flow Transducers, Sensors Offer Fast Response Times, Electronic Design News, May 25, 1989, pages 55-68.
The problem with the above-described sensors is their limited dynamic range. Typically sensors are rated for a range of flow-rates, for example one sensor might be rated for the range from 0 to 100 mL per minute while another sensor may be rated from 0 to 10 mL per minute. The reason for this rating is that such previous sensors are subject to saturation. Saturation occurs when the flow of fluid through the mass-flow sensor is such that the heat exchange time constant between the sensor and the fluid is exceeded. In other words, as fluid moves faster and faster through the sensor, a point is reached where less rather than more differential heat is transferred to the downstream and upstream temperature sensors. This results in a decrease in the temperature difference between the sensors. While such a decrease in differential temperature is normally indicative of a decrease in mass-flow, in fact, mass-flow is increasing.
Consequently, a need exists for a mass-flow sensor having a wide dynamic range so that a single sensor could be used in multiple applications as compared to previously dedicated sensors for particular application mass flow ranges.
The present invention overcomes the problems of the past and provides a mass-flow sensor having a wide dynamic range of operation, in part, by providing a sensor element which in one embodiment is controlled in order to maintain a constant average resistance, while the temperature and resistance widely varies across the sensor element. In that embodiment, the difference in resistance between the first and second halves of the sensor element is a measure of mass flow.
It will be noted that U.S. Pat. No. 4,449,401 Kaiser, et al. discloses the use of a heated sensor 108. It is stated that the current through the sensor is adjusted to keep the temperature, and thus resistance at a single location in the mass-flow, constant. The current required to maintain constant resistance is said to be directly related to massflow. In order to determine mass-flow, a matched sensor used to determine ambient temperature is required. The differences between the device of Kaiser, et al. and the present invention will be appreciated from the following description.